The diversity of the cytochromes P-450 in human tissues, i.e. liver and placenta was investigated using monoclonal antibodies (MAbs) to 3-methylcholanthrene (MC) and phenobarbital (PB)-incduced rat liver cytochrome P-450. The content of epitope-specific cytochrome P-450 was determined by competitive radioimmunoassay (RIA) using a panel of MAbs. The isozymic content varied among different individuals and tissues. The content of antigenically unique types of cytochrome P-450 can therefore be measured in different individuals and thus provide important information on their capacity for drug activation to toxic metabolites and their susceptibility to chemical carcinogens.